Change the brush of a grease pencil object
It's possible but you don't have much control. Here is how you can do it.
First Select your grease pencil object and go into draw mode and select the brush type you want it to be redrawn with. Then go into object mode and select your grease pencil object and right click and select convert to bezier curve from the context menu. Select the generated bezier curve usually named Lines. then right click and select convert to grease pencil object.